The present invention relates in general to printing and, in particular, to a device for detecting a proportion of printed surface on an original pattern for use in a printing machine. The detected value of the printed surface proportion is used for a preliminary adjustment of ink dosing operations of inking units of the printing machine.
A device for measuring the surface range of an image to be printed on an offset printing plate is known from the German publication DE-OS No. 30 29 273. This prior art device optically scans the printing plate by means of a photo-electric detecting device having the shape of a straight line. To illuminate the printing plate, there are provided two linear light sources arranged in parallel one to another. The light sources are equally spaced from the surface of the printing plate to be measured and light emitted from the light sources is deviated by lateral light screens to direct the light on a desired surface section or range.
This known arrangement produces more accurate measurements even in the case when the upper surface of the printing plate is not even. However, the disadvantage of the prior art device is in that the light sources illuminate also the surface ranges adjacent to the range under measurement and the marginal regions of the measured range are double illuminated from one side only. The resulting measuring error thus increases in the direction toward the rims of the printing plate. As a consequence, the determination of the presetting values for the ink metering is inaccurate and a loss in printing quality may occur.